Compact and lightweight radiography apparatus



Sept 2, 1969 J. H. DADLER-RACZ 3,465,152

COMPACT AND LGHTWEIGHT RADIOGRAPHY APPARATUS Filed Oct. 3. 1966 United States Patent O ,3,465,152 COMPACT AND LIGHTWEIGHT RADIOGRAPHY APPARATUS Joseph H. dAdler-Racz, Tilf, Belgium, assignor to Usines Balteau Societe Anonyme, Liege, Belgium Filed Oct. 3, 1966, Ser. No. 583,827 Claims priority, applicationgFrance, Oct. 15, 1965,

8 Int. Cl. H01j 35/16 U.S. Cl. Z50-90 5 Claims ABSTRACT F THE DISCLOSURE The present invention relates to a radiography apparatus constituted by a sealed enclosure in which there is located an X-ray tube which, from a high voltage transformer, is sup-plied with rectified HT through rectifier devices which embody semiconductors and capacitors.

It is well known that X-ray tubes perform best, i.e. will take the highest volta-ge, the highest load and give the longest working life, when supplied with a D.C. voltage The simplest way to obtain a high D.C. voltage is through the medium of a high voltage transformer, rectifiers and filtering capacitors; however, these elements make the equipment bulky, heavy and expensive.

It is also well known for X-ray tubes to be supplied with high A.C. voltages; here, the current passes through the tube in one direction only since the tube itself has a valve action; in this case, electrostatic charges build up on the glass, meaning that the tube is highly loaded during the negative half-wave of the A.C. supply voltage. To avoid the danger of perforation of the insulation, it is therefore necessary to limit the supply voltage and/ or the current passed by the tube.

This difficulty is overcome by providing a rectified supply without filtering. This means that the high voltage applied to the tube is a pulse voltage, i.e. does not change polarity. An X-ray tube operated in this fashion has a performance almost as good as that of a tube supplied with a D.C. voltage. Given this advantage, X- ray equipment has already been designed in which the supply voltage is economically rectified, in particular using vacuum tubes and semiconductor devices. Unfortunately, the use of semiconductors necessitates a bridge arrangement for proper rectification and, in the case of an X-ray tube operating at a very high voltage it is necessary to use a huge number of series-connected rectifier elements since a single such element can take only a few tens of volts. Thus, for example, if an X-ray tube operating at a voltage of 40() kv. is supplied with a rectified voltage by dry rectifiers, around 30,000 rectifier elements will -be needed.

The accommodation of this huge number of rectifier plates in the sealed enclosure constitutes a tremendous problem and attempts have already been made to find a simple and economical solution, however, these attempts have always come up against the problem of the considerable bulk of the devices. Moreover, these rectifier plates have a considerable overall weight to which there must be added the weight of their insulation, particular care having to be taken in the latter respect due to the rL ICC very high voltages involved. These drawbacks stemming from the weight and bulk of the devices have always stood in the way of the creation of a radiography equipment of small size and low weight which could be easily transported.

The present invention, which is designed to overcome these drawbacks, relates to a radiography apparatus in which the X-ray tube situated in a sealed enclosure containing an insulating medium, is supplied with a high rectified voltage by semiconductor rectifiers connected in a circuit which also includes capacitors, is characterised in that the capacitors are constituted by the metallic protective covering of the said X-ray tube, by at least two metal components connected in each case to the electrodes of the said tube, and by the insulating filling of the equipment, the thus formed capacitors being arranged to have a capacitance falling between two predetermined limits; and in that the said capacitance should be sufficiently low in order that, at the end of the positive half wave the said capacitor shall be virtually discharged, and sufficiently high in order that the rectifier current during thenegative half wave, shall not produce across the terminals of the said capacitors a voltage such that there could develop across the terminals of the X-ray tube a voltage which might jeopardize the insulation of said tube.

In a preferred embodiment, the metal components connected to the electrodes of the X-ray tube are constituted by metal parts provided for supporting the tube, similar to the caps which cover the ends of the tube.

Further details will be apparent from the ensuing description of the attached drawings which, simply by way of illustration, show an embodiment of the invention.

The figure presents a schematic view of a radiography apparatus in accordance with the invention, and of itS electrical circuitry.

The apparatus illustrated, operates at a voltage of 400 kv. and at current of 2 ma.; the voltage `across the terminals of the X-ray tube is a pulse voltage.

In a sealed enclosure 2, there is located an X-ray tube 7 around which there is a protective shield 6 which is X- ray opaque. This shield contains a window 6a through which the X-rays can pass, and is integral with the metal support frame 11 fixed to the enclosure 2 which is X-ray transparent. The shield is grounded since the enclosure 2 itself is grounded, for example through a conductor 2a. At its ends, the enclosure 2 carries handles 1 by which the apparatus can be lifted and moved.

In the tube 7, the electrodes 7a and 7b are located. The cathode 7b is constituted by a filament heated by the current from a transformer 12, the latter supplied with a voltage 220 v. at 50 c./s. through the conductors 14. The HT supplied to the tube 7 is provided from the two groups of dry rectifiers 4a and 4b which are connected to two high voltage transformers 3a and 3b supplied with a low voltage by the conductors 13 and 1S respectively. The two transformers 3a and 3b and the two groups of rectifiers 4a and 4b, are arranged in series at either side of the tube 7.

The lsealed enclosure 2 is completely filled by an insulating gas in the form of sulphur hexafiuoride at a pressure in the order of 21/2 atmosphere.

At its two ends, the X-ray tube 7 is provided with metal support caps 5a and 5b which are connected to the corresponding electrodes 7a and 7b. These supports 5a, 5b are attached to insulating arms 10 themselves attached to a metal support frame 11 fixed inside the enclosure 2. The capacitance of the capacitors formed by the metal support pieces 5a and 5b, and the protective shield 6, has a value which is predetermined by the dimensioning of the component elements.

Thus, in the case ofthe apparatus described, which contains a 400 kv. tube, the metal supports a and Sb have a diameter of around 100 mm. and a length of 100 mm., while the protective shield 6 has an internal diameter of 200 mm. In this way, the surfaces of the protective shield 6 and of the two metal support pieces 5a and 5b opposite, constitute two capacitors the respective capacitances of which satisfy the conditions set out in accordance with this invention. In fact, these capacitances are 8 pf. Thus, during the positive half wave of the HT supplied to the tube 7, the latter carries a current of 2 ma. and the capacitors, charged up to 200 kv., discharge in 0.5 ms. while the time taken for the positive voltage to collapse is around 5 ms. It will thus be seen that the capacitive arrangement discharges sufiiciently rapidly and that when the tube ceases to pass current the metal support pieces or caps 5a and 5b are at a potential near ground potential and the capacitors which they constitute together with the protective shield 6, are virtually discharged. On the other hand, during the negative half wave of the HT, the X-ray tube 7 passes no current. The rectifiers 4 which always pass some reverse current, allow a leakage current of about 30 microamps which will charge up each of the capacitors to give a maximum electrical charge of 30 micro-amps x 5 ms., i.e. 150 nanocoulornbs. This means that the potential on the metallic support components 5a and 5b will be no more than in relation to ground.

It will be seen therefore that this is only about a tenth of the maximum voltage carried by an electrode. Thus, during the negative half wave the tube carries only about a tenth of the maximum voltage and the actual voltage It should be noted that if the capacitance of the capacitors is much below 8 pf., the rectiiiers will all be at the potential of the transformers and consequently the full voltage will be applied across the tube during the negative half wave. This effect, which would prejudice the proper operation of the tube, is avoided thanks to the invention.

In practice, the X-ray tube will cease to conduct current before the positive half wave of the HT reaches zero, so that during the negative half wave the tube carries a small positive voltage or a voltage near zero. The result of this is that the tube operates under less severe conditions and consequently its working life is considerably increased.

A surprising but highly advantageous effect of the invention is that each group of rectiiiers carries a maximum voltage of only 200 kv. the practical result of which is that the number of rectifier plates required is reduced by half so that the apparatus described contains around 15,000 plates whereas in known systems around 30,000 are needed.

What I claim is:

1. A radiography apparatus comprising an enclosure, an insulating medium in said enclosure, a metal support frame fixed to said enclosure, insulating arms attached to said metal support frame, metal support pieces fixed to said insulating arms, an X-ray tube fixed in said metal support pieces, electrodes in said X-ray tube comprising an anode and cathode each connected to one of said metal support pieces, a protective shield around said X- ray and fixed to said metal support frame, a window in said shield and in said enclosure, a heating transformer connected to said cathode and fixed in said enclosure, a circuit in said enclosure for supplying said X-ray tube with rectified high voltage, said circuit comprising high Voltage transformers, one side of each of said transformers being connected to a low voltage source of current, the other side of cach of said transformers being connected to dry rectifiers, said rectiers in turn being connected to said anode and cathode and to two capacitors, said capacitors being constituted by said metal shield of said X-ray tube and by at least said metal support pieces connected to said respective electrodes of said X-ray tube, and by said insulating medium in said enclosure, and having a capacitance which is both, sutiiciently low so that said capacitors are virtually discharged at the completion of the positive half-wave, and is also sufficiently high to ensure that during the negative halfwave a leakage current of the said rectifier does not charge up the capacitors suliciently to produce across the terminals of the capacitors a voltage, the magnitude of which when appearing across the terminals of the X-ray tube, would be sufficient to damage the insulation of the tube.

2. Radiography apparatus as claimed in claim 1, characterised in that said metal pieces connected to the electrodes of the X-ray tube are constituted by the supports which locate the ends of the said tube.

3. Radiography apparatus as claimed in claim 2, characterised in that said pieces supporting the ends of the X-ray tube are in the form of caps which at least partially envelop the said ends.

4. Radiography apparatus as claimed in claim 2, characterised in that said pieces supporting the ends of the X-ray tube are separate pieces.

5. Radiography apparatus as claimed in claim 2, characterised in that said pieces supporting the ends of the X-ray tube are integral with said tube.

References Cited UNITED STATES PATENTS 2,216,210 10/ 1940 Mutscheller 250-90 2,925,499 2/ 1960 Seidel 250-90 WILLIAM F. LINDQUIST, Primary Examiner U.S. Cl. X.R. 250-98 

